Fatty acids from carbohydrates

The human body is capable of synthesizing some fatty acids from carbohydrates or other fatty acids. However, humans cannot synthesize sufficient amounts of polyunsaturated fatty acids such as linoleic acid, linolenic acid, and arachidonic acid. Because they must be obtained from the diet, they are known as essential fatty acids. In infants, a deficiency of essential fatty acids can cause skin dermatitis. However, the role of fatty acids in adult nutrition is not well understood. Adults do not usually have a deficiency of essential fatty acids. 

Lipid metabolism in the liver is closely linked to the carbohydrate and amino acid metabolism. When there is a good supply of nutrients in the resorptive (wellfed) state (see p. 308), the liver converts glucose via acetyl CoA into fatty acids. The liver can also take up fatty acids from chylomicrons, which are supplied by the intestine, or from fatty acid-albumin complexes (see p. 162). Fatty acids from both sources are converted into fats and phospholipids. Together with apoproteins, they are packed into very-low-density lipoproteins (VLDLs see p.278) and then released into the blood by exocytosis. The VLDLs supply extrahepatic tissue, particularly adipose tissue and muscle. 

Malonyl-CoA, the first intermediate in the cytosolic biosynthesis of long-chain fatty acids from acetyl-CoA (see Fig. 21-1), increases in concentration whenever the animal is well supplied with carbohydrate excess glucose that cannot be oxidized or stored as glycogen is converted in the cytosol into fatty acids for storage as triacylglycerol. The inhibition of carnitine acyltrans-ferase I by malonyl-CoA ensures that the oxidation of... 

T Biosynthesis and degradation of triacylglycerols are regulated such that the favored path depends on the metabolic resources and requirements of the moment. The rate of triacylglycerol biosynthesis is profoundly altered by the action of several hormones. Insulin, for example, promotes the conversion of carbohydrate to triacylglycerols (Fig. 21-19). People with severe diabetes mellitus, due to failure of insulin secretion or action, not only are unable to use glucose properly but also fail to synthesize fatty acids from... 

When the diet contains more fatty acids than are needed immediately as fuel, they are converted to triacylglycerols in the liver and packaged with specific apolipoproteins into very-low-density lipoprotein (VLDL). Excess carbohydrate in the diet can also be converted to triacylglycerols in the liver and exported as VLDLs (Fig. 21-40a). In addition to triacylglycerols, VLDLs contain some cholesterol and cholesteryl esters, as well as apoB-100, apoC-I, apoC-II, apoC-III, and apo-E (Table 21-3). These lipoproteins are transported in the blood from the liver to muscle and adipose tissue, where activation of lipoprotein lipase by apoC-II causes the release of free fatty acids from the VLDL triacylglycerols. Adipocytes take up these fatty acids, reconvert them to triacylglycerols, and store the products in intracellular lipid droplets myocytes, in contrast, primarily oxidize the fatty acids to supply energy. Most VLDL remnants are removed from the circulation by hepatocytes. The uptake, like that for chylomicrons, is... 

Of the 20 amino acids the human body uses to build proteins, the adult body is able to produce 12 of them in amounts sufficient for its needs—it produces these amino acids from carbohydrates and fatty acids. The remaining 8, listed in Table 13.6, must be obtained from food. Because the body needs these eight amino acids but cannot synthesize them, they are called essential amino acids, in the sense that it is essential we get adequate amounts of them from our food. To support rapid growth, infants and children require, in addition to the eight amino acids listed for adults in Table 13.6, large amounts of arginine and histidine, which can be obtained only from the diet. Infants and juveniles therefore have a total of 10 essential amino acids. (The term essential is unfortunate because, in truth, all 20 amino acids are vital to our good health.)... 

Plants make oils for energy storage in seeds. Because plants must synthesize all their cellular components from simple inorganic compounds, plants—but usually not animals—can use fatty acids from these oils to make carbohydrates and amino acids for later growth after germination. 

In addition to long-chain fatty acids from plasma, the major nutrients utilized for milk fat synthesis are glucose, acetate and 0-hydroxybutyrate. Kinetics for the uptake of these from blood were reported by Miller et al. (1991). Glucose is absolutely required for milk synthesis, being a precursor for lactose or other carbohydrates, or both, in all terrestrial mammals (Oftedal and Iverson, 1995). 

Lipids have an adverse effect on carbohydrate metabolism under basal conditions. The infusion of 20% triglyceride emulsion with heparin during basal insulin and glucose turnover conditions resulted in a rise of plasma free fatty acids from 0.4 to 0.8 mmol/1 at a low rate of infusion (0.5 ml/minute for 2 hours) to between 1.6 and... 

Starch likewise adsorbs fatty acid from an alcohol medium but not from a hydrocarbon medium, " and Lehrman has shown that this action follows a typical Freundlich adsorption isotherm. It is possible that fatty acid can be carried into the interior of the granule only by those solvents which are sufficiently hydrophilic to penetrate the carbohydrate lattice or the mechanism may involve the replacement of adsorbed water in the granule by alcohol, and the subsequent replacement of this alcohol by fatty acid. 

The USPNF 23 describes lecithin as a complex mixture of acetone-insoluble phosphatides that consists chiefly of phosphatidylcholine, phosphatidylethanolamine, phosphatidylser-ine, and phosphatidylinositol, combined with various amounts of other substances such as triglycerides, fatty acids, and carbohydrates as separated from a crude vegetable oil source. 

During prolonged starvation or when carbohydrate metabolism is severely impaired, as in uncontrolled diabetes mel-iitus (see Chapter 25), the formation of acetyl-CoA exceeds the supply of oxaioacetate. The abundance of acetyl-CoA results from excessive mobilization of fatty acids from adipose tissue and excessive degradation of the fatty acids by p-oxidation in the liver. The resulting acetyl-CoA excess is diverted to an alternative pathway in the mitochondria and forms acetoacetic acid, P-hydroxybutyric acid, and acetone—three compounds known collectively as ketone bodies (Figure 26-9). The presence of ketone bodies is a frequent finding in severe, uncontrolled diabetes melUtus. 

For a given physiologic environment, the heart selects the most efficient substrate for energy production. A fitting example is the switch from fatty acid to carbohydrate oxidation with an acute work jump or increase in workload.15 The transient increase in rates of glycogen oxidation is followed by a sustained increase in rates of glucose and lactate oxidation (Fig. 2). Because oleate oxidation remains unaffected by the work jump, the increase in 02 consumption and cardiac work are entirely accounted for by the increase in carbohydrate oxidation. 

---